The hydrocarbonylation of butadiene to obtain 3-pentenoic ester is a valuable process since 3-pentenoic ester is the starting material for the preparation of 5-formylvalerate esters, which are important intermediates for the preparation of caprolactam. Processes for preparing 3-pentenoic ester from hydrocarbonylation of butadiene are disclosed in U.S. Pat. No. 5,028,734 and U.S. Pat. No. 5,495,041. Both processes involve using butadiene to undergo the hydrocarbonylation reaction, providing 3-pentenoic ester as the major product. The catalyst for the hydrocarbonylation of butadiene disclosed in U.S. Pat. No. 5,028,734 is composed of a palladium complex, bidentate phosphine ligands and a benzoic carboxylic acid. The catalyst composition converts 75% of the butadiene to 3-pentenoic ester. The disadvantage of the catalyst is that another side reaction product, 2-pentenoic ester, is produced in addition to the 3-pentenoic ester in the reaction mixture. Thereby, the selectivity for 3-pentenoic ester using this catalyst composition is low. The other disadvantage of the invention is that the catalyst composition comprises benzoic acid, which can easily undergo esterfication in the presence of alcohols, thereby necessitating a constant feeding of the benzoic acid into the catalyst system to maintain efficient reactivity. Moreover, the co-catalyst can not be reused before hydrolysis and recovery. The hydrocarbonylation catalyst disclosed in U.S. Pat. No. 5,495,041 is composed of a palladium complex, bindenate phosphine ligands and 3-pentenoic acid. The drawback of this invention is that the conversion rate of butadiene is about 80%; however, the selectivity of 3-pentenoic ester is less than 50%. Therefore, the yield of 3-pentenoic esters according to the invention is too low.
Both catalyst systems suffer from the disadvantage of that either extra benzoic acid must be constantly fed into the catalyst system to maintain sufficient catalytic activity or the selectivity of 3-pentenoic esters is low. Consequently, it is necessary to develop a catalyst composition which converts butadiene to give 3-pentenoic ester with high selectivity and which undergos hydrocarbonylation reaction without the requirement of constant feeding of co-catalyst.